Optical information recoding medium and manufacturing method thereof

ABSTRACT

The optical information recording medium of the present invention includes a plurality of information layers provided on a substrate and an optical separating layer provided between information layer adjacent to each other, and information is recorded or reproduced by irradiation of a laser beam. When an information layer that is provided closest to a laser beam incident side of the plurality of information layers is taken as a first information layer and an optical separating layer provided in contact with the first information layer is taken as a first optical separating layer, then the first information layer comprises a recording layer, a transmittance adjusting layer that adjusts a transmittance of the first information layer, and a low refractive index layer provided between the transmittance adjusting layer and the first optical separating layer.

TECHNICAL FIELD

The present invention relates to an optical information recording mediumhaving a multilayered structure in which information can be recorded,erased, rewritten and reproduced optically on a plurality of informationlayers by irradiation of laser beams or the like and a method formanufacturing the same.

BACKGROUND ART

In a phase change optical information recording medium used as arewritable medium, information can be recorded, erased and rewritten byutilizing a recording layer in which phase change is caused reversiblybetween a crystalline phase and an amorphous phase. When such arecording layer is irradiated with a laser beam at a high power and thencooled, the irradiated portion becomes an amorphous phase. Furthermore,when the amorphous portion of the recording layer is irradiated with alaser beam at a low power and then gradually cooled, the irradiatedportion becomes a crystalline phase. Therefore, in the phase changeoptical information recording medium, the recording layer can be changedarbitrarily into the amorphous phase or the crystalline phase byirradiating the recording layer with a laser beam whose power ismodulated between a high power level and a low power level. In such anoptical information recording medium, information is recorded byutilizing a difference between the reflectance in the amorphous phaseand the reflectance in the crystalline phase.

In recent years, in order to improve the recording density of theoptical information recording medium, various techniques have beenresearched. For example, the following techniques have been researched:a technique by which a smaller recording mark is recorded by using ablue-violet laser beam having a relatively short wavelength; and atechnique by which a smaller recording mark is recorded by reducing thethickness of a substrate provided on the laser beam incident side andusing a lens having a large numerical aperture (NA). Moreover, there isa technique by which two information layers, each of which include arecording layer, are provided, and information is recorded andreproduced on each of the two information layers by using a laser beamincident from one side (e.g., JP2000-36130A).

FIG. 4 shows an example of a structure of a conventional opticalinformation recording medium in which two information layers areprovided. In the conventional optical information recording medium 101,a first information layer 103, an optical separating layer 104 and asecond information layer 105 are provided in this order from the laserbeam incident side between a first substrate 102 and a second substrate106. In the first information layer 102, a protective layer 1031, arecording layer 1032, a protective layer 1033, a reflective layer 1034and a transmittance adjusting layer 1035 are provided in this order fromthe laser beam incident side.

Thus, in the optical information recording medium 101 in whichinformation is recorded and reproduced in the two information layers byirradiation of a laser beam from one side, recording and reproduction ofthe second information layer 105 provided on the side opposite to thelaser beam incident side is performed by using a laser beam that hasbeen transmitted through the first information layer 103 provided on thelaser beam incident side.

In order to perform recording and reproduction in such an opticalinformation recording medium 101, it is preferable that the firstinformation layer 103 has a transmittance as high as possible.Therefore, in the first information layer 103, a transmittance adjustinglayer 1035 made of a dielectric material having a high refractive indexis provided in contact with the reflective layer 1034 in order toincrease the transmittance. As a dielectric material having a highrefractive index, for example, TiO₂, Nb₂O₅ and materials containingthese can be used.

The optical information recording medium 101 provided with thetransmittance adjusting layer 1035 as described above generally ismanufactured in the following process order to facilitatefilm-formation.

-   (a) a process of forming the second information layer 105 on the    second substrate 106.-   (b) a process of forming the optical separating layer 104 on the    second information layer 105.-   (c) a process of forming the first information layer 103 on the    optical separating layer 104.-   (d) a process of attaching the first substrate 102 onto the first    information layer 103.

In order words, in the conventional optical information recording medium101, when forming the first information layer 103 on the opticalseparating layer 104 in the process (c), first, the transmittanceadjusting layer 1035 is formed on the optical separating layer 104,using a dielectric material having a high refractive index.

However, when the inventors produced the transmittance adjusting layer1035 with a single-wafer film-forming apparatus for single waferprocessing having a plurality of film-formation chambers, the followingwas made evident. When forming the transmittance adjusting layer 1035with a dielectric material having a high refractive index in a firstfilm-formation chamber, since the dielectric material having a highrefractive index is very sensitive to an atmosphere for film formation,the film-forming rate tends to be varied by the influence of, forexample, water contained in a base material (the state in which thesecond information layer 105 and the optical separating layer 104 areformed on the second substrate 106). When a load lock chamber, which isthe first chamber when a base material is introduced to a film-formationchamber, is kept in a vacuum for a long time in order to solve thisproblem, the variation in the film-forming rate can be suppressed.However, in view of productivity, this is not preferable because when avacuum is kept for a long time, the film-forming cycle takes a longtime.

DISCLOSURE OF INVENTION

An optical information recording medium of the present inventioncomprises a substrate, a plurality of information layers provided on thesubstrate, and an optical separating layer provided between theinformation layers adjacent to each other, in which information isrecorded or reproduced by irradiation of a laser beam is characterizedin that when an information layer provided closest to the laser beamincident side of the plurality of information layers is taken as a firstinformation layer, and the optical separating layer provided in contactwith the first information layer is taken as a first optical separatinglayer. The first information layer comprises a recording layer that canchange between two optically different states, a transmission adjustinglayer that adjusts the transmittance of the first information layer, anda low refractive index layer provided between the transmittanceadjusting layer and the first optical separating layer.

A method for manufacturing an optical information recording medium ofthe present invention is a method for manufacturing an opticalinformation recording medium including at least a first informationlayer and a second information layer that are laminated via an opticalseparating layer, the method comprising:

-   (a) a step of forming the second information layer,-   (b) a step of forming the optical separating layer on the second    information layer, and-   (c) a step of forming the first information layer on the optical    separating layer,

wherein the step (c) comprises a step of forming a low refractive indexlayer on the optical separating layer, a step of forming a transmittanceadjusting layer on the low refractive index layer and a step of forminga recording layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing one embodiment of an opticalinformation recording medium of the present invention.

FIG. 2 is a cross-sectional view showing more specifically the structureof the optical information recording medium of FIG. 1.

FIG. 3 is a view showing schematically the structure of a portion of arecording/reproducing apparatus for recording/reproducing informationon/from the optical information recording medium of the presentinvention.

FIG. 4 is a cross-sectional view showing an example of the structure ofa conventional optical information recording medium including twoinformation layers.

BEST MODE FOR CARRYING OUT THE INVENTION

In the optical information recording medium of the present invention, alow refractive index layer is provided between a transmission adjustinglayer and a first optical separating layer in the first informationlayer provided closest to the laser beam incident side. Therefore, sincea variation in the film-forming rate of the transmittance adjustinglayer can be suppressed, the transmittance adjusting layer can be formedstably. Thus, an optical information recording medium that has goodrecording sensitivity and can provide a sufficient C/N, although it hasa plurality of information layers, can be provided.

In the optical information recording medium of the present invention,when the refractive index of the low refractive index layer with respectto the laser beam is taken as n1 and the refractive index of the firstoptical separating layer is taken as n4, it is preferable that n1 and n4satisfy a relationship |n1−n4|≦0.5, more preferably, |n1−n4|≦0.1. Thisis because satisfactory reflectance characteristics can be obtained.

In the optical information recording medium of the present invention,the recording layer contained in the first information layer is formedof a material that can change between a crystalline state and anamorphous state, and when the transmittance of the first informationlayer with respect to the laser beam when the recording layer is in acrystalline state is taken as Tc1 (%) and the transmittance of the firstinformation layer with respect to the laser beam when the recordinglayer is in an amorphous state is taken as Ta1 (%), it is preferablethat Tc1 and Ta1 satisfy a relationship: 40<Tc1, and 40<Ta1. This isbecause a laser beam can reach the information layer provided fartherthan the first information layer when viewed from the laser beamincident side in a sufficient amount of light.

In the optical information recording medium of the present invention,the first information layer further includes a reflective layer providedbetween the recording layer and the transmittance adjusting layer, andwhen the refractive index of the transmittance adjusting layer withrespect to the laser beam is taken as n2 and the extinction coefficientthereof is taken as k2, and the refractive index of the reflective layerwith respect to the laser beam is taken as n3 and the extinctioncoefficient thereof is taken as k3, it is preferable that at least oneof the following relationships is satisfied.1.0≦(n2−n3)≦3.0, and1.0≦(k3−k2)≦4.0.This is because light is confined in the transmittance adjusting layerhaving a larger refractive index and a smaller extinction coefficientthan those of the reflective layer, so that the interference effect oflight becomes large, and therefore the transmittance of the firstinformation layer can be increased.

In the optical information recording medium of the present invention, itis preferable that the low refractive index layer contains at least oneselected from SiO₂, Al₂O₃, LaF₃, ZrSiO₄ and ZrO₂. This is because thesematerials have a small difference in the refractive index with respectto the materials generally used for the optical separating layer and arestable.

In the optical information recording medium of the present invention, itis preferable that the thickness of the low refractive index layer is 1nm or more and 25 nm or less. This is because a reduction in the overallfilm-forming cycle time due to the formation of the low refractive indexlayer can be suppressed.

According to the method for manufacturing the optical informationrecording medium of the present invention, the transmittance adjustinglayer is formed after the low refractive index layer is formed on theoptical separating layer, so that a variation in the film-forming rateof the transmittance adjusting layer is suppressed, and thus thetransmittance adjusting layer can be formed stably. Thus, an opticalinformation recording medium that has good recording sensitivity and canprovide a sufficient C/N, although it has a plurality of informationlayers, can be manufactured.

In the method for manufacturing the optical information recording mediumof the present invention, with respect to a laser beam used whenrecording or reproducing information, the refractive index of the lowrefractive index layer formed in the process (c) described above istaken as n1, and the refractive index of the optical separating layer istaken as n4, it is preferable that the low refractive index layer andthe optical separating layer are formed so that n1 and n4 satisfy arelationship:|n1−n4|≦0.5.This is because an optical information recording medium havingsatisfactory reflectance characteristics can be produced.

In method for manufacturing the optical information recording medium ofthe present invention, it is preferable to form the low refractive indexlayer with a material containing at least one selected from SiO₂, Al₂O₃,LaF₃, ZrSiO₄ and ZrO₂ in the process (c). This is because thesematerials have a small difference in the refractive index with respectto the materials generally used for the optical separating layer and arestable.

In the method for manufacturing the optical information recording mediumof the present invention, it is preferable that the low refractive indexlayer is formed to a film thickness of 1 nm or more and 25 nm or less inthe process (c). This is because a reduction in the overall film-formingcycle time due to the formation of the low refractive index layer can besuppressed.

Hereinafter, embodiments of the present invention will be described morespecifically with reference to the accompanying drawings. However, thepresent invention is not limited by these drawings.

EMBODIMENT 1

One embodiment of the optical information recording medium will bedescribed. The optical information recording medium of the presentinvention is a recording medium including a plurality of informationlayers that are separated from each other by the optical separatinglayer. In this embodiment, an optical information recording mediumincluding two information layers will be described, for example.

FIG. 1 is a cross-sectional view in the radial direction showing aschematic multilayered structure of a disk-shaped optical informationrecording medium 1 (hereinafter, referred to as “an optical disk 1”) ofthis embodiment. As shown in FIG. 1, a first substrate 11, a firstinformation layer 12, an optical separating layer (first opticalseparating layer) 13, a second information layer 14 and a secondsubstrate 15 are laminated in this order in the optical disk 1. Each ofthe two information layers 12, 14 that are laminated via the opticalseparating layer 13 includes a recording layer (not shown), andinformation is recorded in these two information layers 12, 14.

FIG. 2 shows an example of a specific structure of the first informationlayer 12 and the second information layer 14 in the optical disk 1. Alaser beam 2 used for recording or reproducing information is incidenton the optical disk 1 from the first substrate 11 side. In the firstinformation layer 12, a lower protective layer 121, a lower interfacelayer 122, a recording layer 123, an upper protective layer 124, areflective layer 125, a transmittance adjusting layer 126 and a lowrefractive index layer 127 are formed sequentially in this order fromthe incident side of the laser beam 2. In the second information layer14, a lower protective layer 141, a recording layer 142, an upperprotective layer 143, and a reflective layer 144 are formed in thisorder sequentially from the incident side of the laser beam. In thenaming for the interface layers and the protective layers, “lower” meansthat the layer is provided on the incident side of the laser beam 2 withrespect to the recording layer, and “upper” means that the layer isprovided on the side opposite to the incident side of the laser beam 2with respect to the recording layer. As a method for forming each layercontained in the first information layer 12 and the second informationlayer 14, in general, electron beam evaporation, sputtering, CVD(Chemical Vapor Deposition), laser sputtering or the like can be used.

Hereinafter, each layer contained in the optical disk 1 will bedescribed more specifically.

For the materials of the optical separating layer 13 and the firstsubstrate 11, resins such as light-curing resin (in particular,UV-curing resin) or slow acting heat-curing resin can be used. Theoptical separating layer 13 and the first substrate 11 also can beformed by laminating a plurality of resins. For the materials of theoptical separating layer 13 and the first substrate 11, it is preferablethat the optical absorption with respect to the laser beam 2 used issmall, and that the birefringence is optically small in a shortwavelength region. For the first substrate 11, a transparent disk-shapedresins such as polycarbonate, amorphous polyolefin or polymethylmethacrylate (PMMA), or glass can be used. In this case, the firstsubstrate 11 can be formed by being attached to the lower protectivelayer 121 of the first information layer 12 with a resin such as alight-curing resin (in particular, UV-curing resin) or slow actingheat-curing resin.

The second substrate 15 is a disk-shaped substrate. In the secondsubstrate 15, for example, resins such as polycarbonate, amorphouspolyolefin or PMMA, or glass can be used. Guide grooves for guiding thelaser beam 2 may be formed, if necessary, on the surface of the secondsubstrate 15 on the side of the second information layer 14. In thesecond substrate 15, it is preferable that the surface on the sideopposite to the second information layer 14 side is smooth. For thematerial for the second substrate 15, polycarbonate is particularlypreferable because of its excellent properties for transfer and massproduction and its low cost. It is preferable that the thickness of thesecond substrate 15 is in the range from 400 μm to 1300 μm so thatsufficient strength is provided and the thickness of the optical disk 1is about 1200 μm. In the case where the thickness of the first substrate11 is about 600 μm (which is a thickness that allows satisfactoryrecording/reproduction at NA=0.6), it is preferable that the thicknessof the second substrate 15 is in the range from 550 μm to 650 μm. In thecase where the thickness of the first substrate 11 is about 100 μm(which is a thickness that allows satisfactory recording/reproduction atNA=0.85), it is preferable that the thickness of the second substrate 15is in the range from 1150 μm to 1250 μm.

According to the above-described structure, even with irradiation of thelaser beam 2 from one side, information can be recorded/reproducedon/from the second information layer 14 with the laser beam 2transmitted through the first information layer 12.

It should be noted that either one of the first information layer 12 andthe second information layer 14 may be a read-only (ROM (Read OnlyMemory)) information layer or a write-once (WO (Write-Once)) informationlayer that can be written only once.

In the case of high density recording, the wavelength λ of the laserbeam 2 is preferably 450 nm or less, in particular, because the diameterof a spot obtained when the laser beam 2 is focused is determined by thewavelength λ (as the wavelength λ is smaller, the light can be focusedinto a smaller spot). Moreover, if the wavelength λ is less than 350 nm,then the optical absorption with the optical separating layer 13 or thefirst substrate 11 for example is increased. For these reasons, it ismore preferable that the wavelength λ is in the range from 350 nm to 450nm.

Next, each layer constituting the first information layer 12 will bedescribed in detail.

The lower protective layer 121 is formed of a dielectric material. Thefunction of this lower protective layer 121 is to prevent oxidation,corrosion and deformation of the recording layer 123, to adjust theoptical distance so as to increase the optical absorption efficiency ofthe recording layer 123, and to increase the signal amplitude byincreasing the change in the amount of reflected light before and afterrecording. For the lower protective layer 121, for example, oxides suchas SiO_(x) (where x is 0.5 to 2.5), Al₂O₃, TiO₂, Ta₂O₅, ZrO₂, ZnO, andTe—O can be used. Furthermore, nitrides such as C—N, Si—N, Al—N, Ti—N,Ta—N, Zr—N, Ge—N, Cr—N, Ge—Si—N, or Ge—Cr—N can be used. Moreover,sulfides such as ZnS or carbides such as SiC also can be used. Moreover,it is also possible to use a mixture of the above materials. Forexample, ZnS—SiO₂, which is a mixture of ZnS and SiO₂, is particularlyexcellent as the material of the lower protective layer 121. This isbecause ZnS—SiO₂ is an amorphous material, which has a high refractiveindex, fast film formation speed, and favorable mechanical propertiesand resistance against moisture.

The thickness of the lower protective layer 121 can be determinedprecisely so as to satisfy the conditions that increase the change inthe amount of the reflected light between when the recording layer 123is in a crystalline phase and when it is in an amorphous phase andincrease the transmittance of the first information layer 12 bycalculation based on the matrix method (e.g., see the third chapter“Wave Optics”, written by Hiroshi Kubota, Iwanami Shoten, 1971).

The upper protective layer 124 has the function to adjust the opticaldistance to increase the optical absorption efficiency of the recordinglayer 123, and the function to increase the signal amplitude byincreasing the change in the amount of reflected light before and afterrecording. The upper protective layer 124 can be made using an oxidesuch as SiO₂, Al₂O₃, Bi₂O₃, Nb₂O₅, TiO₂, Ta₂O₅, ZrO₂, and ZnO. It alsocan be made using a nitride, such as C—N, Si—N, Al—N, Ti—N, Ta—N, Zr—N,Ge—N, Cr—N, Ge—Si—N, Ge—Cr—N or Nb—N. Moreover, sulfides such as ZnS,carbides such as SiC or C also can be used. Moreover, it is alsopossible to use a mixture of the above materials. When a nitride is usedfor the upper protective layer 124, the upper protective layer 124serves to promote crystallization of the recording layer 123. In thiscase, materials containing Ge—N are preferable because they are formedeasily by reactive sputtering, and have excellent mechanical propertiesand resistance against moisture. Of these, in particular, compositenitrides such as Ge—Si—N, or Ge—Cr—N are preferable. Furthermore,ZnS—SiO₂, which is a mixture of ZnS and SiO₂, is an amorphous material,and has a high refractive index, fast film formation speed, andfavorable mechanical properties and resistance against moisture, andtherefore also is an excellent material for the upper protective layer124.

The film thickness d5 of the upper protective layer 124 is preferably inthe range of ( 1/64)λ/n5≦d5≦ (¼)λ/n5, and more preferably (1/64)λ/n5/d5≦(⅛) λ/n5, where n5 is the refractive index of the upperprotective layer 124. It should be noted that for example, when thewavelength λ of the laser beam 2 and the refractive index n5 of theupper protective layer 124 satisfy 350 nm ≦λ≦450 nm, 1.5≦n5≦3.0, thefilm thickness d5 is preferably in the range of 2 nm≦d5≦75 nm, morepreferably 2 nm≦d5≦40 nm. By choosing the film thickness d5 from thisrange, it is possible to diffuse heat generated in the recording layer123 effectively to the side of the reflective layer 125.

The transmittance adjusting layer 126 is made of a dielectric material,and has the function to adjust the transmittance of the firstinformation layer 12. With this transmittance adjusting layer 126, it ispossible to increase both the transmittance Tc (%) of the firstinformation layer 12 when the recording layer 123 is in the crystallinephase and the transmittance Ta (%) of the first information layer 12when the recording layer 123 is in the amorphous phase. Morespecifically, in the first information layer 12 provided with thetransmittance adjusting layer 126, the transmittance is increased byabout 2% to 10%, compared with the case without the transmittanceadjusting layer 126. Moreover, the transmittance adjusting layer 126also has the function to diffuse heat generated in the recording layer123 effectively.

It is preferable that the refractive index n2 and the extinctioncoefficient k2 of the transmittance adjusting layer 126 satisfy 2.0≦n2and k2≦0.1, more preferably 2.0≦n2≦3.0 and k2≦0.05, in order to increasethe effect of enhancing the transmittances Tc and Ta of the firstinformation layer 12.

It is preferable that the film thickness d2 of the transmittanceadjusting layer 126 is within the range of ( 1/32)λ/n2≦d2≦( 3/16)λ/n2 or( 17/32)λ/n2≦d2≦( 11/16)λ/n2, and more preferably within the range of (1/16)λ/n2≦d2≦( 5/32)λ/n2 or ( 9/16)λ/n2≦d2≦( 21/32)λ/n2. It should benoted that when the wavelength λ of the laser beam 2 and the refractiveindex n2 of the transmittance adjusting layer 126 are set to, forexample, 350 nm≦λ≦450 nm and 2.0≦n2≦3.0, then it is preferable that thefilm thickness d2 is in the range of 3 nm≦d2≦40 nm or 60 nm≦d2≦130 nm,and more preferably in the range of 7 nm≦d2≦30 nm or 65 nm≦d2≦120 nm. Bychoosing the film thickness d2 from these ranges, both transmittances Tcand Ta of the first information layer 12 can be enhanced.

The transmittance adjusting layer 126 can be made using, for example,oxides such as TiO₂, ZrO₂, ZnO, Nb₂O₅, Ta₂O₅, or Bi₂O₃. It also can bemade using a nitride, such as Ti—N, Zr—N, Nb—N, Ta—N, Si—N, Ge—N, Cr—N,Al—N, Ge—Si—N, or Ge—Cr—N. It is also possible to use a sulfide, such asZnS. Moreover, it is also possible to use a mixture of the abovematerials. Of these, in particular TiO₂ or a material including TiO₂ asthe main component is used preferably. These materials have a highrefractive index (n2=2.5 to 2.8) and a low extinction coefficient(k2=0.0 to 0.05), so that the effect of enhancing the transmittance ofthe first information layer 12 is large. Moreover, the transmittanceadjusting layer 126 further may contain an oxide such as SiO₂ or Al₂O₃.

The low refractive index layer 127 is made of a dielectric, and has thefunction to prevent moisture adsorbed to a base material (in the statebefore the transmittance adjusting layer 126 is formed) on which thetransmittance adjusting layer 126 is to be formed from entering afilm-formation chamber when forming the transmittance adjusting layer126. When forming the transmittance adjusting layer 126 made of adielectric material having a high refractive index by sputtering, thisdielectric material having a high refractive index is very sensitive toan atmosphere for film formation. Therefore, when the low refractiveindex layer 127 is not provided, the film-forming rate tends to bevaried by the influence of moisture or the like contained in the opticalseparating layer 13, but the variation in the film-forming rate can besuppressed by providing the low refractive index 127, as in thisembodiment.

Since the low refractive index layer 127 does not have to have anoptical function, it is preferable that the difference between therefractive index n1 of the low refractive index layer 127 and therefractive index n4 of the optical separating layer 13 in contacttherewith is small, and it is preferable to satisfy |n1-n4|≦0.5.Furthermore, in order to provide the low refractive index layer 127having a sufficient thickness, it is preferable that |n1-n4|≦0.1 issatisfied, that is, the difference in the refractive index is evensmaller.

In a single-wafer film-forming apparatus, the overall film-formingthroughput is determined by the rate that is limited by the rate in achamber with the longest film-formation time. The film thickness dl ofthe low refractive index layer 127 is preferably in the range from 1 nmor more and 25 nm or less in order not to reduce the overallfilm-forming throughput, and more preferably in the range from 5 nm ormore and 15 nm or less. By choosing the film thickness d1 in the rangeabove, the low refractive index layer 127 that effectively preventsoxygen from the base material from affecting the atmosphere in thefilm-formation chamber when forming the transmittance adjusting layer126 without limiting the film-forming throughput by the rate for the lowrefractive index layer 127 can be provided.

The low refractive index layer 127 can be made using, for example, SiO₂,Al₂O₃, LaF₃, ZrSiO₄, or ZrO₂. Moreover, it is also possible to use amixture of the above materials. Of these, in particular SiO₂ or amaterial including SiO₂ is used preferably. The refractive indexes ofthese materials are 1.4 to 1.6, and there is no large difference fromthe refractive index of the material generally used for the opticalseparating layer 13, and they are stable materials, and therefore aresuitable as the material for the low refractive index layer 127.

The function of the lower interface layer 122 is to prevent themigration of substances between the lower protective layer 121 and therecording layer 123 due to repeated recording. The lower interface layer122 can be made using nitrides such as C—N, Ti—N, Zr—N, Nb—N, Ta—N,Si—N, Ge—N, Cr—N, Al—N, Ge—Si—N, or Ge—Cr—N, oxides such as Cr₂O₃, oroxynitride containing these systems. Moreover, it can be made using C.Of these, materials containing Ge—N can be formed easily by reactivesputtering and become an interface layer having excellent mechanicalproperties and resistance against moisture. Of these, in particular,composite nitrides such as Ge—Si—N, or Ge—Cr—N are preferable. When theinterface layer is thick, the reflectance or the absorption of the firstinformation layer 12 is changed significantly so as to affect therecording/erasing performance. Therefore, it is preferable that the filmthickness of the interface layer is in the range from 1 nm to 10 nm,more preferably in the range from 2 nm to 5 nm.

An upper interface layer further may be provided at the interfacebetween the recording layer 123 and the upper protective layer 124. Inthis case, the upper interface layer can be made using the materialsdescribed with respect to the lower interface layer 122. It ispreferable that the film thickness thereof is in the range from 1 nm to10 nm (more preferably in the range from 2 nm to 5 nm) for the samereason as with the lower interface layer 122.

Interface layers may be arranged between the upper protective layer 124and the reflective layer 125 and between the reflective layer 125 andthe transmittance adjusting layer 126. These interface layers have thefunction to prevent migration of substances between the upper protectivelayer 124 and the reflective layer 125 and between the reflective layer125 and the transmittance adjusting layer 126 under high temperature andhigh humidity and in recording. In this case, the interface layers canbe made using the materials described with respect to the lowerinterface layer 122. It is preferable that the film thickness thereof isin the range from 1 nm to 10 nm (more preferably in the range from 2 nmto 5 nm) for the same reason as with the lower interface layer 122.

It is sufficient that the recording layer 123 contains a substance thatcan change between the crystalline state and the amorphous state (phasechange material), and can be formed of a phase change materialincluding, for example, Te, In or Se as the main component. Examples ofthe main component of well-known phase change materials includeTe—Sb—Ge, Te—Ge, Te—Ge—Sn, Te—Ge—Sn—Au, Sb—Se, Sb—Te, Sb—Se—Te, In—Te,In—Se, In—Se—Tl, In—Sb, In—Sb—Se, and In—Se—Te. Of these, it was provedby investigating with experiments for materials that have favorableproperties of repeated recording and erasure that compositions of thesematerials including three elements Ge, Sb, and Te as the main componentsare preferable. Furthermore, it also was proved that when the ratio ofthe atomic weight of these elements is expressed by Ge_(x)Sb_(y)Te_(z),the composition where 0.1≦x≦0.6, y≦0.5, 0.4≦z≦0.65 (where x+y+z=1) isparticularly preferable.

In the optical disk 1 in this embodiment, it is necessary to make thefilm thickness of the recording layer 123 as small as possible toincrease the transmittance of the first information layer 12 in orderthat the amount of laser light necessary for recording/reproductionreaches the second information layer 14. It is preferable that the filmthickness of the recording layer 123 is in the range of 3 nm to 9 nm,and even more preferably in the range of 4 nm to 8 nm.

The reflective layer 125 has the optical function to increase the amountof light that is absorbed by the recording layer 123. The reflectivelayer 125 also has the thermal function to quickly diffuse heat that isgenerated in the recording layer 123 to allow easier amorphization ofthe recording layer 123. Furthermore, the reflective layer 125 also hasthe function to protect the multi-layer film from the environment inwhich it is used.

As the material of the reflective layer 125, it is possible to use asingle metal with a high thermal conductivity, such as Ag, Au, Cu or Al.Moreover, an alloy including one or a plurality of these metal elementsas the main component and to which one or a plurality of other elementsare added in order to, for example, improve resistance against moistureor adjust thermal conductivity can be used. More specifically, alloyssuch as Al—Cr, Al—Ti, Au—Pd, Au—Cr, Ag—Pd, Ag—Pd—Cu, Ag—Pd—Ti, Ag—Ru—Auor Cu—Si can be used. In particular Ag alloys have a high thermalconductivity and a high transmittance of light, so that they arepreferable as the material for the reflective layer 125.

It is preferable that the refractive index n3 and the extinctioncoefficient k3 of the reflective layer 125 satisfy n3≦2.0, and 1.0≦k3,and more preferably 0.1≦n3≦1.0, and 1.5≦k3≦4.0 in order further toincrease the transmittance of the first information layer 12.

In order to make the transmittance Tc and the Ta of the firstinformation layer 12 as high as possible, it is preferable that the filmthickness d3 of the reflective layer 125 is in the range from 3 nm to 15nm, more preferably 8 nm to 12 nm. When the film thickness d3 of thereflective layer 125 is 3 nm or more, a sufficient thermal diffusionfunction can be obtained, and sufficient reflectance for the firstinformation layer 12 can be obtained. Furthermore, when the filmthickness d3 of the reflective layer 125 is 15 nm or less, sufficienttransmittance of the first information layer 12 can be obtained.

It is preferable that the refractive index n2 and the extinctioncoefficient k2 of the transmittance adjusting layer 126 and therefractive index n3 and the extinction coefficient k3 of the reflectivelayer 125 satisfy 1.0≦ (n2−n3)≦3.0 or 1.0≦(k3−k2)≦4.0, more preferably2.0≦(n2−n3)≦3.0 or 1.5≦ (k3−k2)≦3.0. When these relationships aresatisfied, light is confined in the transmittance adjusting layer 126having a larger refractive index and a smaller extinction coefficientthan those of the reflective layer 125, and the interference effect oflight becomes large, so that the transmittance of the first informationlayer 12 can be increased. For example, when TiO₂ is used as thetransmittance adjusting layer 126 and an Ag alloy is used as thereflective layer 125, n2=2.7, k2=0.0, n3=0.2, and k3=2.0 at a wavelengthof 405 nm, and (n2−n3)=2.5 and (k3−k2)=2.0. Thus, the above relationshipcan be satisfied.

The optical separating layer 13 has the function to discriminate thefocus position of the first information layer 12, in addition to thefunction to separate optically the first information layer 12 and thesecond information layer 14. It is necessary that the thickness of theoptical separating layer 13 is equal to at least a focal depth ΔZ thatis determined by the numerical aperture NA of the objective lens and thewavelength λ of the laser beam 2. When it is assumed that the referenceintensity at the focal point is 80% of that in the case of noaberration, ΔZ can be approximated to ΔZ=k/{2(NA)²}. When Δ=400 nm andNA=0.6, ΔZ=0.556 μm, and the focal depth is within ±0.6 μm. Therefore,in this case, it is necessary that the thickness of the opticalseparating layer 13 is 1.2 μm or more. It is preferable that thedistance to the first information layer 12 is in the range in which thelaser beam 2 can be focused using an objective lens. Therefore, it ispreferable that the total of the thickness of the optical separatinglayer 13 is within a tolerance (e.g., 50 μm or less) that can be allowedby the objective lens.

A guide groove for guiding the laser beam 2 may be formed on the surfaceof the optical separating layer 13 on the incident side of the laserbeam 2, if necessary.

In order for the amount of laser light necessary forrecording/reproduction to reach the information layer on the sideopposite to the first information layer 12 when viewed from the incidentside of the laser beam 2, it is preferable that the transmittances Tcand Ta of the first information layer 12 satisfy 40<Tc1, and 40<Ta1, andmore preferably 43<Tc1 and 43<Ta1.

It is preferable that the transmittances Tc and Ta of the firstinformation layer 12 satisfy −5≦ (Tc1−Ta1)≦5, and more preferably−3≦(Tc1−Ta1)≦3. When Tc1 and Ta1 satisfy these conditions, an effect ofa change of the transmittance due to the state of the recording layer123 of the first information layer 12 is small duringrecording/reproduction of the second information layer 14, and goodrecording/reproduction characteristics can be obtained.

It is preferable that the reflectances Rc1 and Ra1 of the firstinformation layer 12 satisfy Ra1<Rc1. By doing this, the reflectance ishigh in an initial state in which information is not recorded, so that arecording/reproducing operation can be performed stably. Furthermore, itis preferable that Rc1 and Ra1 satisfy 0.1≦Ra1≦5 or 4≦Rc1≦15, morepreferably, 0.5≦Ra1≦3 or 4≦Rc1≦10 so that a difference in thereflectance (Rc1-Ra1) can be large so that good recording/reproductioncharacteristics can be obtained.

Next, the structure of the second information layer 14 will be describedin detail. The second information layer 14 is formed of a lowerprotective layer 141, a recording layer 142, an upper protective layer143 and a reflective layer 144 that are arranged in this order from theincident side of the laser beam 2. In the second information layer 14,information is recorded/reproduced by the laser beam 2 that has passedthrough the first substrate 11, the first information layer 12 and theoptical separating layer 13.

The lower protective layer 141 is formed of a dielectric material as thelower protective layer 121. The lower protective layer 141 has thefunction is to prevent oxidation, corrosion and deformation of therecording layer 142, to adjust the optical distance in order to increasethe optical absorption efficiency of the recording layer 142, and toincrease the signal amplitude by increasing the change in the amount ofreflected light before and after recording. The lower protective layer141 can be made using, for example, oxides such as SiO_(x) (where x is0.5 to 2.5), Al₂O₃, TiO₂, Ta₂O₅, ZrO₂, ZnO, and Te-O can be used. Italso can be made using a nitride, such as C—N, Si—N, Al—N, Ti—N, Ta—N,Zr—N, Ge—N, Cr—N, Ge—Si—N, or Ge—Cr—N. Moreover, sulfides such as ZnS orcarbides such as SiC also can be used. Moreover, it is also possible touse a mixture of the above materials. As in the case of the lowerprotective layer 121, ZnS—SiO₂ is particularly excellent as the materialof the lower protective layer 141.

The film thickness of the lower protective layer 141 can be determinedprecisely so as to satisfy the conditions that increase the change inthe amount of the reflected light between when the recording layer 142is in a crystalline phase and when it is in an amorphous phase andincrease the transmittance of the first information layer 12 bycalculation based on the matrix method as in the case of the lowerprotective layer 121.

The upper protective layer 143 has the function to adjust the opticaldistance in order to increase the optical absorption efficiency of therecording layer 142, and the function to increase the carrier level byincreasing the change in the amount of reflected light before and afterrecording, as in the case of the upper protective layer 124. The upperprotective layer 143 can be made using an oxide such as SiO₂, Al₂O₃,Bi₂O₃, Nb₂O₅, TiO₂, Ta₂O₅, ZrO₂, and ZnO, as in the case of the upperprotective layer 124. It also can be made using a nitride, such as C—N,Si—N, Al—N, Ti—N, Ta—N, Zr—N, Ge—N, Cr—N, Ge—Si—N, Ge—Cr—N or Nb—N.Moreover, sulfides such as ZnS, carbides such as SiC or C also can beused. Moreover, it is also possible to use a mixture of the abovematerials. When a nitride is used for the upper protective layer 143,the upper protective layer 143 serves to promote crystallization of therecording layer 123, as in the case of the upper protective layer 124.In this case, materials containing Ge—N are preferable because they areformed easily by reactive sputtering, and have excellent mechanicalproperties and resistance against moisture. Of these, in particular,composite nitrides such as Ge—Si—N or Ge—Cr—N are preferable.Furthermore, ZnS—SiO₂ also is an excellent material for the upperprotective layer 143 as in the case of the upper protective layer 124.

An interface layer may be provided at the interface between therecording layer 142 and the upper protective layer 143 or the recordinglayer 142 and the lower protective layer 141. In this case, for theinterface layer, the materials described with reference to the lowerinterface layer 122 can be used. For the same reason as with the lowerinterface layer 122, it is preferable that the film thickness is in therange from 1 nm to 10 nm, (more preferably 2 nm to 5 nm).

The material of the recording layer 142 in this embodiment is made of amaterial that changes the phase reversibly between the crystal phase andthe amorphous phase by irradiation of the laser beam 2, as in the caseof the recording layer 123. The recording layer 142 is made of a phasechange material including, for example, Te, In or Se as the maincomponent, as in the case of the recording layer 123. Examples of themain component of well-known phase change materials include Te—Sb—Ge,Te—Ge, Te—Ge—Sn, Te—Ge—Sn—Au, Sb—Se, Sb—Te, Sb—Se—Te, In—Te, In—Se,In—Se—Tl, In—Sb, In—Sb—Se, and In—Se—Te. Of these, it was proved byinvestigating with experiments for materials that have favorablerepeated rewriting properties of recording and erasure and thecompositions of these materials that materials including three elementsGe, Sb, and Te as the main components are preferable. Furthermore, whenthe ratio of the atomic weight of these elements is expressed byGe_(x)S_(y)Te_(z), the composition where 0.1≦x≦0.6, y≦0.5, 0.4≦z≦0.65,(where x+y+z=1) is particularly preferable.

In order to increase the recording sensitivity of the second informationlayer 14, it is preferable that the film thickness of the recordinglayer 142 is in the range of 6 nm to 20 nm. Even in this range, when therecording layer 142 is thick, heat is diffused in the in-plane directionso that thermal influence on an adjacent region becomes large, and whenthe recording layer 142 is thin, the reflectance of the secondinformation layer 14 becomes small. Therefore, the film thickness of therecording layer 142 is more preferably in the range of 9 nm to 15 nm.

The reflective layer 144 has the optical function to increase the amountof light that is absorbed by the recording layer 142, as in the case ofthe reflective layer 125. Furthermore, as in the case of the reflectivelayer 125, the reflective layer 144 also has the thermal function todiffuse quickly heat that is generated in the recording layer 142, andto allow easier amorphization of the recording layer 142. Furthermore,the reflective layer 144 also has the function to protect themulti-layer film from the environment in which it is used, as in thecase of the reflective layer 125.

As the material of the reflective layer 144, it is possible to use asingle metal with a high thermal conductivity, such as Ag, Au, Cu or Al,as in the case of the reflective layer 125. More specifically, alloyssuch as Al—Cr, Al—Ti, Au—Pd, Au—Cr, Ag—Pd, Ag—Pd—Cu, Ag—Pd—Ti, Ag—Ru—Auor Cu—Si can be used. In particular, Ag alloys have a high thermalconductivity and a high transmittance of light, so that they arepreferable as the material for the reflective layer 144. The secondinformation layer 14 is an information layer positioned farthest fromthe incident side of the laser beam. Therefore, the second informationlayer 14 does not need a high transmittance, so that it is preferablethat the film thickness of the reflective layer 144 is 30 nm or more,which provides a sufficient thermal diffusion function. Even in thisrange, when the reflective layer 144 is thicker than 200 nm, the thermaldiffusion function becomes too large, and the recording sensitivity ofthe second information layer 14 is decreased. Therefore, the filmthickness of the reflective layer 144 is preferably in the range from 30nm to 200 nm.

A metal layer may be provided at the interface between the upperprotective layer 143 and the reflective layer 144. In this case, thematerial described with reference to the reflective layer 144 can beused for the metal material. The film thickness is preferably in therange from 3 nm to 100 nm (more preferably 10 nm to 50 nm).

EMBODIMENT 2

One embodiment of a method for manufacturing an optical informationrecording medium of the present invention will be described. In thisembodiment, a method for manufacturing the optical disk 1 (see FIG. 2)described in Embodiment 1 will be described.

First, an second information layer 14 is formed on a second substrate15. More specifically, first, a second substrate 15 (thickness is, forexample, 1100 μm) is prepared, and is disposed in a film-formationapparatus.

Then, a reflective layer 144 is formed on the second substrate 15. Thereflective layer 144 can be formed by sputtering using a sputteringtarget including elements constituting the reflective layer 144 in an Argas atmosphere or an atmosphere of a mixed gas of Ar gas and a reactivegas (at least one gas selected from oxygen gas and nitrogen gas). Inthis case, when a guide groove for guiding the laser beam 2 to thesecond substrate 15 is formed, the reflective layer 144 is formed on thesurface on the side in which the guide groove is formed.

Then, an upper protective layer 143 is formed on the reflective layer144. The upper protective layer 143 can be formed by sputtering using asputtering target including elements constituting the upper protectivelayer 143 in an Ar gas atmosphere or an atmosphere of a mixed gas of Argas and a reactive gas.

Then, a recording layer 142 is formed on the upper protective layer 143.Furthermore, an interface layer is formed between the upper protectivelayer 143 and the recording layer 142, if necessary.

The recording layer 142 can be formed by sputtering using a sputteringtarget made of a material including Te, In, Se or the like as the maincomponent in accordance with its composition, using one power.

For the atmosphere gas (sputtering gas) for sputtering, Ar gas, Kr gas,a mixed gas of Ar gas and Kr gas, a mixed gas of Ar gas and a reactivegas, or a mixed gas of Kr gas and a reactive gas can be used.

As described in Embodiment 1, the film thickness of the recording layer142 is preferably in the range from 6 nm to 20 nm, more preferably inthe range from 9 nm to 15 nm. The film-forming rate of the recordinglayer 142 can be controlled by power introduced from a power source. Ifthe film-forming rate is reduced too much, it takes a long time to forma film, and in addition to that, gas in the atmosphere can enter therecording layer more than necessary. If the film-forming rate isincreased too much, the film-formation time can be short, but it becomesdifficult to control the film thickness precisely. Therefore, thefilm-forming rate of the recording layer 142 is preferable in the rangefrom 0.1 nm/sec. to 6 nm/sec.

Then, a lower protective layer 141 is formed on the recording layer 142.The lower protective layer 141 can be formed by sputtering using asputtering target including elements constituting the lower protectivelayer 141 in an Ar gas atmosphere or an atmosphere of a mixed gas of Argas and a reactive gas. Furthermore, an interface layer is formedbetween the recording layer 142 and the lower protective layer 141, ifnecessary.

Thus, the second information layer 14 is formed. Then, an opticalseparating layer 13 is formed on the lower protective layer 141 of thesecond information layer 14. The optical separating layer 13 can be madeby applying and spin-coating a light-curing resin (in particular aUV-curing resin) or a slow-acting heat-curing resin on the lowerprotective layer 141, and then curing the resin. When the opticalseparating layer 13 is provided with a guide groove of the laser beam 2,the guide groove can be formed by attaching a substrate (mold) fortransfer in which the groove is formed to a resin that is not cured yet,curing the resin, and then detaching the substrate (mold) for transfertherefrom.

If necessary, it is also possible to perform an initialization step ofcrystallizing the entire recording layer 142, after the lower protectivelayer 141 has been formed or after the optical separating layer 13 hasbeen formed. The crystallization of the recording layer 142 can beperformed by irradiating a laser beam 2.

Subsequently, the first information layer 12 is formed on the opticalseparation layer 13. More specifically, first, a base material in whichthe second information layer 14 and the optical separating layer 13 areformed on the second substrate 15 is disposed in a film-formingapparatus, and a low refractive index layer 127 is formed on the opticalseparating layer 13. The low refractive index layer 127 can be formed bysputtering using a sputtering target including elements constituting thelow refractive index layer 127 in an Ar gas atmosphere or an atmosphereof a mixed gas of Ar gas and a reactive gas.

Subsequently, the transmittance adjusting layer 126 is formed on the lowrefractive index layer 127. The transmittance adjusting layer 126 can beformed by sputtering using a sputtering target including elementsconstituting the transmittance adjusting layer 126 in an Ar gasatmosphere or an atmosphere of a mixed gas of Ar gas and a reactive gas.

Subsequently, the reflective layer 125 is formed on the transmittanceadjusting layer 126. The reflective layer 125 can be formed bysputtering using a sputtering target including elements constituting thereflective layer 125 in an Ar gas atmosphere or an atmosphere of a mixedgas of Ar gas and a reactive gas.

Subsequently, the upper protective layer 124 is formed on the reflectivelayer 125. The upper protective layer 124 can be formed by sputteringusing a sputtering target including elements constituting the upperprotective layer 124 in an Ar gas atmosphere or an atmosphere of a mixedgas of Ar gas and a reactive gas.

Subsequently, the recording layer 123 is formed on the upper protectivelayer 124. The recording layer 123 can be formed by sputtering using asputtering target made of a material including Te, In, Se or the like asthe main component in accordance with its composition, using one powersource.

For the sputtering gas atmosphere, it is possible to use Ar gas, Kr gas,a mixed gas of Ar gas and reactive gas, or a mixed gas of Kr gas andreactive gas.

It is preferable that the film thickness of the recording layer 123 isin the range of 3 nm to 9 nm, and even more preferably in the range of 4nm to 8 nm, as described in Embodiment 1. The film-forming rate of therecording layer 123 can be controlled by the power introduced from apower source. When the film-forming rate is reduced too much, it takes along time to form a film, and in addition to that, gas in the atmosphereenters the recording layer 123 more than necessary. When thefilm-forming rate is increased too much, the film forming time can bereduced, but it becomes difficult to control the film thicknessprecisely. Therefore, it is preferable that the film-forming rate of therecording layer 123 is in the range from 0.1 nm/sec to 6 nm/sec.

Subsequently, a low interface layer 122 is formed on the recording layer123, if necessary. The lower interface layer 122 can be formed bysputtering using a sputtering target including elements constituting thelower interface layer 122 in an Ar gas atmosphere or an atmosphere of amixed gas of Ar gas and a reactive gas.

Subsequently, a lower protective layer 121 is formed on the recordinglayer 123 or the lower interface layer 122, if necessary. The lowerprotective layer 121 can be formed in the same manner with the upperprotective layer 124. The composition of the sputtering target used whenforming these protective layers can be selected in accordance with thecomposition of the protective layer and the sputtering gas. In otherwords, these protective layers may be formed using sputtering targetshaving the same composition, or may be formed using sputtering targetshaving different compositions.

Interface layers may be arranged between the upper protective layer 124and the reflective layer 125 and between the reflective layer 125 andthe transmittance adjusting layer 126. The interface layers in this casecan be formed in the same manner with the lower interface layer 122 (thesame applies to the following interface layers).

Finally, the first substrate 11 is formed on the lower protective layer121. The first substrate 11 can be made by applying and spin-coating alight-curing resin (in particular a UV-curing resin) or a slow-actingheat-curing resin on the lower protective layer 121, and then curing theresin. Furthermore, for the first substrate 11, transparent disk-shapedsubstrates made of resins such as polycarbonate, amorphous polyolefin orPMMA, or glass can be used. In this case, a light-curing resin (inparticular a UV-curing resin) or a slow-acting heat-curing resin isapplied on the lower protective layer 121, the substrate is adhered tothe lower protective layer 121 and spin-coating is performed, and thenthe resin is cured to form the first substrate.

It should be noted that, if necessary, it is also possible to perform aninitialization step of crystallizing the entire recording layer 123,after the lower protective layer 121 has been formed or after the firstsubstrate 11 has been formed. The crystallization of the recording layer123 can be performed by irradiating a laser beam 2. Thus, the opticaldisk 1 can be produced as described above.

EMBODIMENT 3

In Embodiment 3, an example of a method for recording/reproducinginformation on/from an optical disk 1 as explained in Embodiment 1 willbe described.

First a recording/reproducing apparatus used for therecording/reproducing method in this embodiment will be described. FIG.3 schematically shows the configuration of a portion of arecording/reproducing apparatus 3 used for a recording/reproducingmethod of this embodiment. The recording/reproducing apparatus 3includes a spindle motor 31 for rotating an optical disk 1, an opticalhead 32 provided with a semiconductor laser 33, and an objective lens 34for focusing the laser beam 2 emitted from the semiconductor laser 33.The optical disk 1 is an optical information recording medium describedin Embodiment 1 and includes two information layers (the firstinformation layer 12 and the second information layer 14). The firstinformation layer 12 includes the recording layer 123, and the secondinformation layer 14 includes the recording layer 142. The objectivelens 34 focuses the laser beam 2 onto the information layers (therecording layer 123 in the case of the first information layer 12, andthe recording layer 142 in the case of the second information layer14.).

Information is recorded, erased and overwritten on the optical disk 1(the first information layer 12 or the second information layer 14) bymodulating the power of the laser beam 2 between the peak power (Pp(mW)) of a high power and the bias power (Pb (mW)) of a low power. Byirradiating a laser beam 2 with the peak power, an amorphous phase isformed in a local portion of the recording layer 123 or 142, and thisamorphous phase serves as a recording mark. Between recording marks, alaser beam 2 with the bias power is irradiated, and a crystalline phase(erased portion) is formed. It should be noted that if the laser beam 2is irradiated with the peak power, then so-called multi-pulses arecommon, in which a pulse train is formed. The multi-pulses may be formedby modulating only with the power levels of the peak power and the biaspower, or they may be formed by modulating with power levels in therange of 0 mW to the peak power.

Moreover, information signals are reproduced by setting as thereproduction power (Pr (mW)) a power that is lower than the power levelof the peak power and the bias power, which does not influence theoptical state of the recording marks when irradiating the laser beam 2with this power level, and with which a sufficient amount of reflectedlight for the reproduction of the recording marks from the optical disk1 can be attained. The signals from the optical disk 1 obtained byirradiating a laser beam 2 with this reproduction power are read with adetector, thus reproducing the information signal.

The numerical aperture (NA) of the objective lens 34 is preferablywithin the range of 0.5 and 1.1 (more preferably within the range of 0.6and 1.0) in order to adjust such that the spot diameter of the laserbeam is within the range of 0.4 μm and 0.7 μm. It is preferable that thewavelength of the laser beam 2 is not greater than 450 nm (morepreferably in the range of 350 nm to 450 nm). It is preferable that thelinear speed of the optical disk 1 when recording information is in therange of 3 m/sec to 20 m/sec (more preferably in the range of 4 m/sec to15 m/sec), because in this range crystallization due to the reproductionlight tends not to occur and a sufficient erasure capability isattained.

When recording information on the first information layer 12, the laserbeam 2 is focused on the recording layer 123, and information isrecorded on the recording layer 123 by the laser beam 2 that has passedthrough the first substrate 11. The reproduction is performed using thelaser beam 2 that has been reflected by the recording layer 123 andpassed through the first substrate 11. When recording information on thesecond information layer 14, the laser beam 2 is focused on therecording layer 142, and information is recorded with the laser beam 2that has passed through the first substrate 11, the first informationlayer 12 and the optical separating layer 13. The reproduction ofinformation is performed using the laser beam 2 that has been reflectedby the recording layer 142 and passed through the optical separatinglayer 13, the first information layer 12 and the first information layer12.

It should be noted that if guide grooves for guiding the laser beam 2are formed in the second substrate 15 and the optical separating layer13, then the recording may be performed on the groove surface (grooves)that is closer to the incident side of the laser beam 2, or on thegroove surface (lands) that is further away therefrom. Information maybe recorded on both the grooves and the lands.

WORKING EXAMPLES

The following is a more detailed explanation of the present inventionusing working examples.

Working Example 1

In Working Example 1, the film-forming rate of the transmittanceadjusting layer when the low refractive index layer was provided wascompared with the film-forming rate of the transmittance adjusting layerwhen the low refractive index layer is not provided. The film-formingrate was measured in the following manner in this example.

Five samples in which the low refractive index layer was provided andfive samples in which the low refractive index layer was not providedwere produced. The samples (samples 1-a, 1-b, 1-c, 1-d, and 1-e) inwhich the low refractive index layer was provided were formed bypreparing a substrate for rate measurement and laminating SiO₂(thickness: 10 nm) as the low refractive index layer and TiO₂(thickness: 20 nm) as the transmittance adjusting layer sequentially onthe substrate by sputtering. The samples (samples 1-f, 1-g, 1-h, 1-i,and 1-j) in which the low refractive index layer was not provided wereformed by preparing a substrate for rate measurement and laminating TiO₂(thickness: 20 nm) as the transmittance adjusting layer on the substrateby sputtering. By measuring the film thickness of each sample, thestability of the film-forming rate of TiO₂ was examined.

Table 1 shows the results of the measurement of the film thickness whenthe low refractive index layer is provided and not provided. Thefilm-forming rate of TiO₂ is in the vicinity of 22.0 Å/sec, andtherefore when ◯ indicates that the rate is within ±1% from 22.0 Å/sec,and Δ indicates that the film-forming rate is within ±3%, and Xindicates that the rate is ±3% or more. TABLE 1 low sample refractivefilm-forming rate variation No. index layer of TiO₂ (Å/sec) evaluationevaluation 1-a present 22.1 ◯ ◯ 1-b present 21.6 Δ 1-c present 22.2 ◯1-d present 21.9 ◯ 1-e present 22.5 Δ 1-f absent 15.6 X X 1-g absent18.6 X 1-h absent 19.9 X 1-i absent 16.7 X 1-j absent 18.8 X

The results confirmed the following. In the samples 1-a, 1-b, 1-c, 1-d,and 1-e that were provided with the low refractive index layer, thefilm-forming rate of TiO₂ was stable and had little variation, and filmscan be formed with sufficiently high reproducibility. On the other, inthe samples 1-f, 1-g, 1-h, 1-i, and 1-j that were not provided with thelow refractive index layer, the film-forming rate of TiO₂ was unstableand had a large variation. The same results were produced when thetransmittance adjusting layer was not TiO₂ and the low refractive indexlayer was not SiO₂, for example, when the transmittance adjusting layerwas Nb₂O₅ and the low refractive index layer was Al₂O₃. The resultsabove confirmed that the configuration in which the low refractive indexlayer is provided is effective to stabilize the film-forming rate of thetransmittance adjusting layer.

Working Example 2

In Working Example 2, the first information layer 12 of the optical disk1 (see FIG. 2) was produced, and the relationship between the refractiveindex n1 and the film thickness d1 of the low refractive index layer 127and the reflectance (Rc1, Rc2) of the first information layer 12 wasinvestigated. More specifically, samples obtained by producing the firstinformation layers 12 having different material and film thickness ofthe low refractive index layer 127, and further forming the firstsubstrate 11 on the first information layer 12 were produced. Regardingthe produced samples, the reflectance of the first information layer 12was measured.

The samples were produced in the following manner. First, apolycarbonate substrate (diameter: 120 mm, thickness: 1100 μm,refractive index: 1.62) was prepared as a substrate. Then, on thispolycarbonate substrate, the low refractive index layer 127, a TiO₂layer (thickness: 20 nm) as the transmittance adjusting layer 126, aAg—Pd—Cu layer (thickness: 10 nm) as the reflective layer 125, aZr—Si—Cr—O layer (thickness: 10 nm) as the upper protective layer 124, aGeSbTe layer (thickness: 6 nm) as the recording layer 123, a Zr—Si—Cr—Olayer (thickness: 5 nm) as the lower interface layer 122, and a ZnS—SiO₂layer (thickness: 40 nm, ZnS: 80 mol %, SiO₂: 20 mol %) as the lowerprotective layer 121 were laminated sequentially by sputtering. An SiO₂layer, an Al₂O₃ layer, a ZrO₂ layer, and a ZnS—SiO₂ layer were used asthe low refractive index layer 127. Finally, the first substrate 11 wasformed by applying a UV-curing resin on the lower protective layer 121,performing spin-coating with a polycarbonate substrate (diameter: 120mm, thickness: 90 μm) adhered to the lower protective layer 121, andthen irradiating UV rays to cure the resin. A plurality of samples formeasuring the reflectance having different materials and thickness ofthe low refractive index layer 127 were produced in this manner.

Regarding the thus obtained samples, first, the reflectance Ra1 (%) inthe case where the recording layer 123 is in an amorphous phase wasmeasured. Thereafter, an initialization process to crystallize therecording layer 123 was performed, and the reflectance Rc1 (%) in thecase where the recording layer 123 is in a crystalline phase wasmeasured. For measurement of the reflectance, the recording/reproducingapparatus 3 shown in FIG. 3 was used. More specifically, the reflectancewas measured by rotating the samples with a spindle motor 31,irradiating and focusing the laser beam 2 with a wavelength of 405 nm onthe recording layer 123 of the first information layer 12 and measuringthe amount of the reflected light.

Table 2 shows the results of the measurement of the reflectance (Rc1,Ra1) of the first information layer 12 in each sample. Table 2 alsoshows the material of the low refractive index layer 127 in each sampleand the absolute value (|n1−n4|) of the difference between therefractive index n1 of the low refractive index layer 127 and therefractive index n4 of the optical separating layer 13 with respect to alaser beam of a wavelength of 405 nm. The refractive index n1 of theSiO₂ layer in a wavelength of 405 nm was 1.49, the refractive index n1of the Al₂O₃ layer in a wavelength of 405 nm was 1.70, the refractiveindex n1 of the ZrO₂ layer in a wavelength of 405 nm was 2.12, and therefractive index n1 of the ZnS—SiO₂ layer in a wavelength of 405 nm was2.34. The refractive index n4 of the transmittance adjusting layer 126in a wavelength of 405 nm was 1.62. For evaluation, ◯ indicates that thereflectance Rc1 in the specular portion of the substrate of the firstinformation layer 12 when the recording layer 123 is in the crystallinephase is in the range 4≦Rc1≦15, and the reflectance Ra1 in the specularportion of the substrate of the first information layer 12 when therecording layer 123 is in the amorphous phase is in the range 0.1≦Ra1≦5,and Δ indicates that either one of them is outside that range. TABLE 2material of sample low refractive eval- No. index layer |n1-n4| d1(nm)Rc1 (%) Ra1 (%) uation 2-a SiO₂ 0.13 1 6.4 1.4 ◯ 2-b SiO₂ 0.13 5 6.2 1.3◯ 2-c SiO₂ 0.13 10 5.9 1.2 ◯ 2-d SiO₂ 0.13 20 5.7 1.0 ◯ 2-e SiO₂ 0.13 255.2 0.9 ◯ 2-f SiO₂ 0.13 30 5.1 0.9 ◯ 2-g Al₂O₃ 0.08 1 6.5 1.5 ◯ 2-hAl₂O₃ 0.08 5 6.6 1.5 ◯ 2-i Al₂O₃ 0.08 10 6.8 1.6 ◯ 2-j Al₂O₃ 0.08 20 7.11.8 ◯ 2-k Al₂O₃ 0.08 25 7.3 1.9 ◯ 2-l Al₂O₃ 0.08 30 7.4 1.9 ◯ 2-m ZrO₂0.50 1 6.7 1.6 ◯ 2-n ZrO₂ 0.50 5 7.8 2.2 ◯ 2-o ZrO₂ 0.50 10 9.3 3.0 ◯2-p ZrO₂ 0.50 20 11.8 4.5 ◯ 2-q ZrO₂ 0.50 25 12.6 4.9 ◯ 2-r ZrO₂ 0.50 3012.9 5.2 Δ 2-s ZnS—SiO₂ 0.72 1 6.8 1.6 ◯ 2-t ZnS—SiO₂ 0.72 5 8.4 2.5 ◯2-u ZnS—SiO₂ 0.72 10 10.5 3.8 ◯ 2-v ZnS—SiO₂ 0.72 20 13.8 5.8 Δ 2-wZnS—SiO₂ 0.72 25 14.6 6.3 Δ 2-x ZnS—SiO₂ 0.72 30 14.9 6.5 Δ

These results show that the samples 2-a, 2-b, 2-c, 2-d, 2-e, and 2-f inwhich the material of the low refractive index layer 127 is made of SiO₂and has a film thickness d1 of 1 nm to 30 nm have a more preferablereflectance satisfying 4≦Rc1≦15, and 0.1≦Ra1≦5.

Furthermore, the samples 2-g, 2-h, 2-i, 2-j, 2-k, and 2-l in which thematerial of the low refractive index layer 127 is made of Al₂O₃ and hasa film thickness d1 of 1 nm to 30 nm have a more preferable reflectancesatisfying 4≦Rc1≦15, and 0.1≦Ra1≦5.

Furthermore, the samples 2-m, 2-n, 2-o, 2-p, and 2-q in which thematerial of the low refractive index layer 127 is made of ZrO₂ and has afilm thickness d1 of 1 nm to 25 nm have a more preferable reflectancesatisfying 4≦Rc1≦15, and 0.1≦Ra1≦5. The reflectance Ra1 of a sample 2-rhaving a film thickness d1 of 30 nm was larger than 5%.

Furthermore, the samples 2-s, 2-t, and 2-u in which the material of thelow refractive index layer 127 is made of ZnS—SiO₂ and has a filmthickness d1 of 1 nm to 10 nm have a more preferable reflectancesatisfying 4≦Rc1≦15, and 0.1≦Ra1≦5. The reflectance Ra1 of samples 2-v,2-w, and 2-x having a film thickness d1 of 20 to 30 nm was larger than5%.

As shown in Table 2, when a material having a large refractive index n1is used for the low refractive index layer 127, Rc1 and Ra1 becomelarge.

The above results confirmed that when the low refractive index layer 127is formed with a film thickness in the range (1 nm to 25 nm) thatprevents the overall film-forming throughput from being reduced, it ispreferable that the refractive index n1 of the low refractive indexlayer 127 and the refractive index n4 of the optical separating layer 13satisfy the relationship |n1−n4|≦0.5 in order to obtain more preferablereflectance characteristics (4≦Rc1≦15, and 0.1≦Ra1≦5). It also wasconfirmed that when a value of |n1−n4| is reduced further, satisfactoryreflectance characteristics can be obtained, even if the film thicknessdl exceeds 25 nm.

INDUSTRIAL APPLICABILITY

According to the optical information recording medium and the method formanufacturing the same of the present invention, a variation in thefilm-forming rate of a transmittance adjusting layer contained in anoptical information recording medium provided with a plurality ofinformation layers can be suppressed, so that the transmittanceadjusting layer can be formed stably. Thus, an optical informationrecording medium that can provide a good recording sensitivity and asufficient C/N ratio, although a plurality of information layers areincluded.

1. An optical information recording medium comprising a substrate, aplurality of information layers provided on the substrate, and anoptical separating layer provided between information layer adjacent toeach other, in which information is recorded or reproduced byirradiation of a laser beam, wherein when an information layer that isprovided closest to a laser beam incident side of the plurality ofinformation layers is taken as a first information layer and an opticalseparating layer provided in contact with the first information layer istaken as a first optical separating layer, the first information layercomprises a recording layer that can change between two opticallydifferent states, a transmittance adjusting layer that adjusts atransmittance of the first information layer, and a low refractive indexlayer provided between the transmittance adjusting layer and the firstoptical separating layer.
 2. The optical information recording mediumaccording to claim 1, wherein when a refractive index of the lowrefractive index layer with respect to the laser beam is taken as n1,and a refractive index of the first optical separating layer is taken asn4, n1and n4 satisfy:|n1−n4|≦0.5.
 3. The optical information recording medium according toclaim 2, wherein n1 and n4 satisfy:|n1−n4|≦0.1.
 4. The optical separating layer according to claim 1,wherein the recording layer contained in the first information layer isformed of a material that can change between a crystalline state and anamorphous state, and when a transmittance of the first information layerwith respect to the laser beam when the recording layer is in thecrystalline state is taken as Tc1(%), and a transmittance of the firstinformation layer with respect to the laser beam when the recordinglayer is in the amorphous state is taken as Ta1(%), Tc1 and Ta1 satisfy:40<Tc1 and 40<Ta1.
 5. The optical information recording medium to claim1, wherein the first information layer further comprises a reflectivelayer provided between the recording layer and the transmittanceadjusting layer, when a refractive index of the transmittance adjustinglayer with respect to the laser beam is taken as n2, an extinctioncoefficient thereof is taken as k2, a refractive index of the reflectivelayer with respect to the laser beam is taken as n3, and an extinctioncoefficient is taken as k3, at least one of the following relationshipsis satisfied:1.0≦(n2−n3)≦3.0 and1.0≦(k3−k2)≦4.0.
 6. The optical information recording medium accordingto claim 1, wherein the low refractive index layer comprises at leastone selected from the group consisting of SiO₂, Al₂O₃, LaF₃, ZrSiO₄, andZrO₂.
 7. The optical information recording medium according to claim 1,wherein the low refractive index layer has a film thickness of 1 nm ormore and 25 nm or less.
 8. A method for manufacturing an opticalinformation recording medium comprising at least a first informationlayer and a second information layer that are laminated via an opticalseparating layer, the method comprising: (a) forming the secondinformation layer, (b) forming the optical separating layer on thesecond information layer, (c) forming the first information layer on theoptical separating layer, wherein the step (c) comprises a step offorming a low refractive index layer on the optical separating layer, astep of forming a transmittance adjusting layer on the low refractiveindex layer and a step of forming a recording layer.
 9. The method formanufacturing the optical information recording medium according toclaim 8, wherein the low refractive index layer and the opticalseparating layer are formed such that when a refractive index of the lowrefractive index layer formed in the step (c) with respect to the laserbeam used for recording or reproducing information is taken as n1, and arefractive index of the first optical separating layer is taken as n4,n1 and n4 satisfy:|n1−n4|≦0.5.
 10. The method for manufacturing the optical informationrecording medium according to claim 8, wherein in the step (c), the lowrefractive index layer is formed of a material comprising at least oneselected from the group consisting of SiO₂, Al₂O₃, LaF₃, ZrSiO₄, andZrO₂.
 11. The method for manufacturing the optical information recordingmedium according to claim 8, wherein in the step (c), the low refractiveindex layer is formed so as to have a film thickness of 1 nm or more and25 nm or less.